To set just a low level for the competence means that any competence at or above this level counts as a match. To set a high level only means that any competence at or below this level counts as a match.
To set no level at all means that any score for this competence counts as a match.
If you select essential for any competence, a person who fails to score a match for this competence over the range you set will not appear in the ranked list at all. Even if the person scores matches for other competencies, the failure to score a match in this essential competence rules him or her out of the list.
If you select desirable, a person who fails to score a match for this competence and range will still be ranked if he or she scores matches in other competencies. If the person scores matches in no competencies he or she will not be listed.
If you select not required, the competence itself does not count for the competence matching exercise. For example, a person scores matches with four competencies in the list. One of these competencies is marked 'not required'. The person's score is shown as three.